Using evidence from those who have specialized knowledge strengthens your argument in two ways. For one, you’re showing that you’re not basing your argument solely on personal opinion, which boosts your credibility. Second, you cite the actual work, often words, of those who have made it their career to study a topic in great depth and therefore can provide reliable information. So, if you’re writing an argument for requiring vaccinations for entrance to college, then evidence from physicians who have researched the topic or someone who works at the U.S. Centers for Disease Control and Prevention could be very effective. In the same way, if you’re writing about national security, providing the viewpoint of someone who serves or has served as secretary of state would add authority to your argument.